The 1988 plastic Australian ten-dollar note employs as an anti-counterfeiting security device, a diffraction grating image of Captain Cook which comprises a regular matrix of pixels each containing a curvilinear segment of one or more of the grating lines. These pixels when illuminated each generate a two-dimensional optical catastrophe image diffraction pattern whereby the total image diffraction pattern of the grating is optically variable but structurally stable. The Captain Cook image is an application of the present inventor's theory of generalised curvilinear diffraction gratings to optical diffraction catastrophes. The theory is outlined in Optlea Acta, 1983, Vol. 30 Nos. 3 and 4, and the application to optical diffraction catastrophes is disclosed in Vol. 30, No. 4, 449-464. In essence, the Captain Cook image entails the imposition of a matrix of diffraction catastrophe pixels on a regular diffraction grating.
By "image diffraction pattern" in the context of this specification is meant the optical image observed by the naked eye focused on the grating when it is illuminated by an arbitrarily extended diffuse source of finite width such as a fluorescent tube. The term "diffracting grating" as employed herein indicates a grating of either reflective or transmissive lines. A pattern is described herein as "optically variable" where it varies according to the position of observation and is "structurally stable" if its broad form at any given position of observation is not materially altered by slight distortions of the grating surface.
The Captain cook pixellated diffraction grating image is substantially more secure that the traditional metal foil insert and is an improvement over security devices such as multiple film devices, conventional straight line grating and image holograms because, in contrast to these devices, it is able to sustain an acceptable level of structural stability as the notes become heavily crinkled in day-to-day use. However, for wider commercial application to credit cards and the like, it would be desirable to be able to easily convert a range of recognisable images, indeed any selected image, to pixellated diffraction gratings. This has not been disclosed to date and is an objective of a first aspect of the present invention.